Memory cell access device having a pn-junction with polycrystalline plug and single-crystal semiconductor regions

ABSTRACT

A memory device includes a driver comprising a pn-junction in the form of a multilayer stack including a first doped semiconductor region having a first conductivity type, and a second doped semiconductor plug having a second conductivity type opposite the first conductivity type, the first and second doped semiconductors defining a pn junction therebetween, in which the first doped semiconductor region is formed in a single-crystalline semiconductor, and the second doped semiconductor region includes a polycrystalline semiconductor. Also, a method for making a memory device includes forming a first doped semiconductor region of a first conductivity type in a single-crystal semiconductor, such as on a semiconductor wafer; and forming a second doped polycrystalline semiconductor region of a second conductivity type opposite the first conductivity type, defining a pn junction between the first and second regions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to U.S. patent application Ser. No.______, entitled MEMORY CELL ACCESS DEVICE HAVING A PN-JUNCTION WITHPOLYCRYSTALLINE AND SINGLE-CRYSTAL SEMICONDUCTOR REGIONS by the sameinventors, filed on the same day as the present application.

BACKGROUND

1. Field of the Invention

The present invention relates to high density memory devices based onphase change based memory materials, including chalcogenide basedmaterials and other programmable resistive materials, and to methods formanufacturing such devices.

2. Description of Related Art

Phase change based memory materials, such as chalcogenide basedmaterials and similar materials, can be caused to change phase betweenan amorphous state and a crystalline state by application of electricalcurrent at levels suitable for implementation in integrated circuits.The generally amorphous state is characterized by higher electricalresistivity than the generally crystalline state, which can be readilysensed to indicate data. These properties have generated interest inusing programmable resistive material to form nonvolatile memorycircuits, which can be read and written with random access.

The change from the amorphous to the crystalline state is generally alower current operation. The change from crystalline to amorphous,referred to as reset herein, is generally a higher current operation,which includes a short high current density pulse to melt or breakdownthe crystalline structure, after which the phase change material coolsquickly, quenching the phase change process and allowing at least aportion of the phase change material to stabilize in the amorphousstate. The magnitude of the current needed for reset can be reduced byreducing the size of the phase change material element in the celland/or the contact area between electrodes and the phase changematerial, such that higher current densities are achieved with smallabsolute current values through the phase change material element.

Because the phase change occurs as a result of heating, a relativelylarge current is needed in order to heat the phase change material andinduce the desired phase change. Field effect transistor access deviceshave been proposed as drivers for phase change memory cells, but fieldeffect transistors (e.g., MOSFET) can have a weaker current drive.Bipolar junction transistors (BJT) can provide larger current drive thanfield effect transistors, but the integration of bipolar junctiontransistors with CMOS peripheral circuitry is difficult and results inhighly complex designs and manufacturing processes.

Diode access devices have been proposed as drivers for phase changememory cells. However, diodes having both regions made of dopedpolysilicon may have an unacceptably high off current. Diodes havingboth regions made of doped single-crystal silicon may provide a suitablylow off current, but processes for making a diode having both regionsmade of doped single-crystal silicon are complex. Diode structures havebeen proposed that include polysilicon for one terminal andsingle-crystal silicon for another. See, U.S. Pat. No. 7,309,921.However, such structures do not completely solve the problem of highoff-current due to the polysilicon terminal, and have not been proposedfor memory cell access devices. See, U.S. Pat. No. 7,157,314.

It is desirable to provide access devices that reliably providesufficient current for phase change memory cell programming while havinga suitably low off current, that are readily manufacturable atacceptable cost, and that are compatible with high performance logiccircuitry.

SUMMARY

In a general aspect a memory device as described herein includes anaccess device that comprises a pn-junction, including a first dopedsemiconductor region having a first conductivity type, and a seconddoped semiconductor region having a second conductivity type oppositethe first conductivity type, the first and second doped semiconductorsdefining a pn-junction therebetween, in which the first dopedsemiconductor region is formed in a single-crystalline semiconductor,and the second doped semiconductor region includes a polycrystallinesemiconductor. The doping concentration in the polysilicon region isgreater than that in the single-crystal region in embodiments describedherein. The second doped semiconductor region is implemented in the formof a plug within a via through an insulating layer, or as a patternedsemiconductor body first patterned and then covered by an insulatinglayer.

In some embodiments the first doped semiconductor region includes alightly-doped P-type semiconductor and the second doped semiconductorregion includes a more heavily-doped N-type polycrystallinesemiconductor having a doping concentration higher than that of thelightly-doped P-type semiconductor; in other embodiments the firstsemiconductor region includes a lightly-doped N-type semiconductor andthe second semiconductor region includes a more heavily-doped P-typepolycrystalline semiconductor. The heavily doped semiconductor regionsin the polysilicon have a doping concentration higher than that of thelightly doped semiconductor regions in the single crystallinesemiconductor so that the electrical junction lies within the singlecrystalline semiconductor when the junction is off, significantlyreducing leakage due to the off state current of the diode. For example,the heavily doped semiconductor regions may have a concentration ofdopants (atoms/cm³), that is more than 10 times the concentration ofdopants in the lightly doped semiconductor regions, and more preferably100 to 1000 times greater or more. For example, the heavily dopedsemiconductor regions have a doping concentration on the order of around10⁺¹⁷ to 10⁺¹⁹/cm³, and the lightly doped semiconductor regions have adoping concentration on the order of 10⁺¹⁴/cm³ to 10⁺¹⁶/cm³.

In some embodiments the single-crystalline semiconductor is asingle-crystal silicon; in some embodiments the polycrystallinesemiconductor is a polysilicon.

In some embodiments the diode further includes an electricallyconductive cap on the second doped semiconductor region; and in somesuch embodiments the cap includes a silicide.

In some embodiments the second doped semiconductor region isself-aligned with the first doped semiconductor region; in someembodiments the second doped semiconductor region is formed as a columnover the first doped semiconductor region.

In some embodiments the memory device further includes a phase changememory element coupled with the second doped semiconductor region.

In another general aspect a memory device array as described hereinincludes first access lines extending in a first direction, and secondaccess lines overlying the first access lines and extending in a seconddirection, and a plurality of memory cells, each comprising an accessdevice and a memory material. The access device includes first andsecond doped semiconductor regions defining a pn-junction. The firstdoped semiconductor region is of a first conductivity type and is formedin a single-crystalline semiconductor substrate, and the first dopedsemiconductor region is in electrical communication with a correspondingfirst access line. The second doped semiconductor region is of a secondconductivity type opposite the first conductivity type and includes apolycrystalline semiconductor. The memory material is in electricalcommunication with a corresponding second access line. In someembodiments the memory material is a phase change memory material.

In some embodiments the access device further includes an electricallyconductive cap on the second doped semiconductor region; and in somesuch embodiments the electrically conductive cap includes a silicide.The memory cell may further include a bottom electrode contacting theelectrically conductive cap, and in such embodiments the memory materialcontacts the bottom electrode. The bottom electrode may be omitted insome embodiments, such as embodiments using a pore type memory cellhaving a pore opening to the cap, filled with programmable resistancematerial. In other embodiments the memory material contacts the secondsemiconductor region.

In some embodiments the diode further includes a thin barrier layer atthe pn-junction. The barrier layer can inhibit diffusion of dopantsacross the pn-junction, and can enhance performance of the diode withoutimpeding on-state current flow in operation of the device.

In some embodiments the memory cell further includes a top electrode,and in such embodiments the memory material contacts the top electrode.In some embodiments the top electrode constitutes the first access line.

In another general aspect a method for making a memory cell diode driveras described herein includes: providing a single-crystallinesemiconductor zone having a first conductivity type; forming aconductively-doped region of a second conductivity type (e.g. an n-well)in the upper region of the semiconductor substrate suitable for actingas an access line; forming a lightly-doped (where lightly-doped is usedin relative sense herein compared to the doping concentration of thepolycrystalline material mentioned below) region of the firstconductivity type within and adjacent the upper surface of theconductively-doped region; forming isolation trenches defining a bottomdiode region strip having an exposed surface; depositing a seconddielectric material onto the surface of the bottom diode region strip;forming a junction via (or contact opening) through the seconddielectric material to expose an area of the lightly-doped region at thesurface of the bottom diode region strip; and forming a moreheavily-doped polycrystalline material of the second conductivity typein the contact opening, the more heavily-doped polycrystalline materialcontacting the area of the lightly-doped region. The polycrystallinematerial may be deposited in the contact opening and then doped; or, thepolycrystalline material may be deposited in a suitably doped form.

The semiconductor wafer substrate may constitute the single-crystallinesemiconductor zone; or, an epitaxially-grown single-crystallinesemiconductor layer formed over an insulation layer on the wafer (“SOIsubstrate”) may constitute the single-crystalline semiconductor zone.

In some embodiments the method further includes forming an electricallyconductive cap over the surface of the heavily-doped polycrystallinematerial.

In another general aspect a method for making a memory cell driver asdescribed herein includes: providing a single-crystalline semiconductorzone having a first conductivity type; forming a conductively-dopedregion of a second conductivity type in the upper region of thesemiconductor substrate; forming a lightly-doped region of the firstconductivity type within and adjacent the upper surface of theconductively-doped region; forming isolation trenches defining a bottomjunction region strip having an exposed surface; depositing a moreheavily-doped polycrystalline material of the second conductivity typeover the surface of the bottom junction region strip; depositing anelectrically conductive cap material over the surface of theheavily-doped polycrystalline material; patterning the cap material andthe more heavily-doped polycrystalline material to form a top junctionelement over the lightly-doped region at the surface of the bottomjunction region strip; depositing a second dielectric material onto thesurface of the first dielectric material and the bottom junction regionstrip and the top diode element; and planarizing the second dielectricmaterial and exposing a portion of the surface of the top junctionelement.

In another general aspect a method for making a memory cell driver asdescribed herein includes: providing a single-crystalline semiconductorsubstrate having a second conductivity type; forming a lightly-dopedregion of the first conductivity type within and adjacent the uppersurface of the substrate; depositing a more heavily-dopedpolycrystalline material of the second conductivity type over thesurface of the lightly-doped region; forming isolation trenches defininga diode strip having an exposed surface; patterning the moreheavily-doped polycrystalline material to isolate a second junctionelement and expose adjacent lightly-doped regions of the firstconductivity type; forming a spacer adjacent the sidewalls of the secondjunction element; forming an electrically conductive cap material overthe second junction element; conducting an implant of the firstconductivity type to more heavily dope the exposed lightly-doped regionsof the first conductivity type to establish conductively doped regions;and depositing a second dielectric material onto the surface of thefirst dielectric material and the second diode element.

In some embodiments of the methods, a thin layer of a barrier material(for example SiO₂ or SiN_(x)O_(y)) is formed on the lightly-doped regionof the first conductivity type prior to depositing the polycrystallinesemiconductor material; as a result a thin barrier is provided at thepn-junction. The barrier layer can inhibit diffusion of dopants acrossthe pn-junction, and can enhance performance of the diode.

Advantageously certain features of the memory array and of peripheraldevices may be formed together. For example, the deposit ofpolycrystalline material may constitute both the polycrystallinesemiconductor node of the diode array and the FET gate in the peripheraldevices. Briefly: the crystalline semiconductor material in the memoryarray region is doped; trench isolation are formed; an oxide is grownover the memory array region and the peripheral device region; the oxideis patterned to form gate oxides for the peripheral devices and toremove the oxide from the memory array; and polycrystalline material isdeposited and patterned for both in the peripheral device region to formgates and in the array area to form polysilicon, more heavily dopedelements of the access devices. Thereafter an interlayer dielectric isformed, and openings are formed in both the peripheral device region andthe memory array region. The openings are filled with a conductivecontact plug material such as tungsten in the peripheral device region,and with elements completing the memory cells in the array region.Therefore, a single polysilicon process can be shared for both the moreheavily doped polysilicon elements in the array and peripheralpolysilicon structures, saving substantial costs in manufacturing. Inthis embodiment, transistor gate structures in the peripheral circuitryregion of the device and the more heavily doped polysilicon elements ofthe drivers comprise respective features in a single polysilicon layer.

Forming the first diode region of single-crystal semiconductor materialprovides for a memory cell having significantly reduced off-current, orleakage current, as compared with a diode having both regions formed ofpolycrystalline material. According to the disclosed embodiments thefirst diode region can be formed in the crystalline semiconductor wafer,and the pn-junction is defined at the wafer surface. It is not necessaryto etch into the wafer to form the pn-junction. Forming the second dioderegion of polycrystalline semiconductor material, rather than ofsingle-crystal semiconductor material simplifies fabrication, because anepitaxial growth procedure by which single-crystal material is typicallyformed is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a memory array employing memory cellshaving diode access devices as described herein.

FIGS. 2A and 2B are diagrammatic sketches in sectional views showing anembodiment of a unit diode access device including a patternedpolysilicon body.

FIG. 2C illustrates a feature of the diode access devices describedherein by which the junction is located primarily in the single crystalportion of the diode.

FIGS. 3 and 4 are diagrammatic sketches in sectional views showingalternative embodiments of a unit diode access device.

FIGS. 5A-5C are diagrammatic sketches in sectional views of anembodiment of memory cells having diode access devices.

FIGS. 6A and 6B are diagrammatic sketches in sectional views of anotherembodiment of memory cells having diode access devices.

FIGS. 7A and 7B are diagrammatic sketches in sectional views of anotherembodiment of memory cells having diode access devices.

FIGS. 8A and 8B are diagrammatic sketches in sectional views of anotherembodiment of memory cells having diode access devices.

FIGS. 9A and 9B are diagrammatic sketches in sectional views of anotherembodiment of memory cells having diode access devices.

FIGS. 10A and 10B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 11A and 11B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 12A and 12B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 13-20C are diagrammatic sketches showing stages in an embodimentof a process for making diode access devices such as are shown forexample in FIGS. 2A, 2B, 3.

FIGS. 21A-25C are diagrammatic sketches showing stages in an embodimentof a process for making diode access devices such as are shown forexample in FIGS. 2A, 2B, 4.

FIGS. 26A and 26B are diagrammatic sketches in sectional views showinganother embodiment of a unit diode access device including a polysiliconplug.

FIGS. 27A and 27B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 28A and 28B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 29A and 29B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 30A and 30B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 31A and 31B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 32A and 32B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 33A and 33B are diagrammatic sketches in sectional views ofanother embodiment of memory cells having diode access devices.

FIGS. 34A-39C are diagrammatic sketches showing stages in an embodimentof another process for making diode access devices such as are shown forexample in FIGS. 26A, 26B.

FIGS. 40A-44C are diagrammatic sketches showing stages in an embodimentof a process for forming memory elements over access diodes such as areshown in FIGS. 2A, 2B, 3, resulting in memory cell devices generally asillustrated in FIGS. 7A, 7B.

FIGS. 45A-50B are diagrammatic sketches showing stages in an embodimentof a process for forming memory elements over access diodes such as areshown in FIGS. 2A, 2B, 4, resulting in memory cell devices generally asillustrated in FIGS. 7A, 7B.

FIGS. 51A-54C are diagrammatic sketches showing stages in an embodimentof a process for forming memory elements over access diodes such as areshown in FIGS. 26A, 26B, resulting in memory cell devices generally asillustrated in FIGS. 27A, 27B.

In the preceding figures, in many instances, the corresponding figureslabeled “A” and “B” (where presented) show sections taken generallyperpendicular to one another; for example, the orientation of thesectional view shown in FIG. 2A is indicated at A-A′ in FIG. 2B, and theorientation of the sectional view shown in FIG. 2B is indicated at B-B′in FIG. 2A. The corresponding figures labeled “C” (where presented) arein a plan view.

DETAILED DESCRIPTION

The invention will now be described in further detail by reference tothe drawings, which illustrate alternative specific embodiments andmethods. The drawings are diagrammatic, showing features of theembodiments and their relation to other features and structures, and arenot made to scale. For improved clarity of presentation, in the figuresillustrating various embodiments, elements corresponding to elementsshown in other drawings are not all particularly renumbered, althoughthey are all readily identifiable in all the figures. Also for clarityof presentation certain features are not shown in the figures, where notnecessary for an understanding of the invention. It is to be understoodthat there is no intention to limit the invention to the specificallydisclosed embodiments and methods but that the invention may bepracticed using other features, elements, methods and embodiments.Preferred embodiments are described to illustrate the present invention,not to limit its scope, which is defined by the claims. Those ofordinary skill in the art will recognize a variety of equivalentvariations on the description that follows.

FIG. 1 illustrates in a schematic diagram of a portion of a memory array100 implemented using memory devices and diode access devices asdescribed herein. In alternatives, access device other than diodes,which also include pn-junctions may be used, such as bipolartransistors. Each of the memory cells of array 100 includes a diodeaccess device and a memory element (represented in FIG. 1 by a variableresistor) capable of being set to one of a plurality of resistive statesand thus capable of storing one or more bits of data.

The array 100 includes a plurality of word lines 130 including wordlines 130 a, 130 b, and 130 c extending in parallel in a firstdirection, and a plurality of bit lines 120 including bit lines 120 a,120 b, and 120 c extending in parallel in a second direction generallyperpendicular to the first direction. The word lines 130 and bit lines120 are arranged typically in such a manner that a given word line 130and a given bit line 120 cross over each other but do not physicallyintersect.

Memory cell 115 is representative of the memory cells of array 100. Thememory cell 115 includes a diode access device 121 and a memory element160 arranged in series; the diode 121 is electrically coupled to theword line 130 b and the memory element 160 is electrically coupled tothe bit line 120 b (or vice-versa).

Reading or writing to memory cell 115 of array 100 can be achieved byapplying appropriate voltages and/or currents to the corresponding wordline 130 b and bit line 120 b to induce a current through a selectedmemory cell 115. The level and duration of the voltages/currents appliedis dependent upon the operation performed, e.g., a reading operation ora writing operation.

In a reset (or erase) operation of memory cell 115 having memory element160 including a phase change material, a reset pulse is applied to thecorresponding word line 130 b and bit line 120 b to cause a transitionof an active region of the phase change material into an amorphousphase, thereby setting the phase change material to a resistance withina resistive value range associated with the reset state. The reset pulseis a relatively high energy pulse, sufficient to raise the temperatureof at least the active region of the memory element 160 above themelting temperature to place at least the active region in a liquidstate. The reset pulse is then quickly terminated, resulting in arelatively quick quenching time as the active region quickly cools tobelow the transition temperature so that the active region stabilizes toan amorphous phase.

In a set (or program) operation of memory cell 115 having memory element160 including a phase change material, a program pulse is applied to thecorresponding word line 130 b and bit line 120 b of suitable amplitudeand duration to induce a current sufficient to raise the temperature ofat least a portion of the active region above the transition temperatureand cause a transition of a portion of the active region from theamorphous phase into a crystalline phase, this transition lowering theresistance of the memory element 160 and setting the memory cell 115 tothe desired state.

In a read (or sense) operation of the data value stored in memory cell115 having memory element 160 comprising phase change material, a readpulse is applied to the corresponding word line 130 b and bit line 120 bof suitable amplitude and duration to induce current to flow that doesnot result in the memory element 160 undergoing a change in resistivestate. The current through the memory cell 115 is dependent upon theresistance of the memory element 160 and thus the data value stored inthe memory cell 115.

FIGS. 2A and 2B show a portion of an embodiment of a memory cell array100, in which the second polysilicon region of the diode has the form ofan island, in diagrammatic sectional views. FIG. 2A is taken in adirection along a bit line 120, and FIG. 2B is taken in a directionalong a word line 130. FIG. 2C illustrates an aspect of the pn-junctionformed as described herein, with a depletion region located mostly inthe crystalline region having a lower concentration of doping. Thisaspect establishes a diode with smaller leakage current in the offcondition, allowing for improved memory operation. Turning first to FIG.2C, using reference numerals that will be applied in FIGS. 2A and 2B, adiode is illustrated including a more heavily doped N+ region 216implemented using polysilicon, and a more lightly doped P− region 214implemented using single crystal silicon. A physical boundary 215between the regions 216 and 214 defines the pn-junction. The width ofthe junction W_(j) however is the sum of the widths of depletion regions215-N and 215-P, which have respective widths labeled W_(N) and W_(P) inthe diagram, where W_(P) is much greater than W_(N). The widths W_(N)and W_(P) of the depletion regions under zero bias are inverselyproportional the doping concentrations, as is known from thepreservation of charge equality:

qN_(A)W_(P)=qN_(D)W_(N),

where q is charge, N_(A) is concentration of acceptors (p-type doping),and N_(D) is concentration of donors (n-type doping). Where N_(D) ismuch greater than N_(A), the depletion region on the p-type material ismuch wider than the depletion region in the n-type material.

So, in embodiments in which the concentration of n-type dopants inregion 216 is 100 times greater than the concentration of p-type dopantsin region 214, the width W_(P) will be 100 times greater than the widthW_(N). As the majority of the junction, as defined by the depletionregions, occurs in the crystalline portion 214 of the diode, theoff-current characteristics are determined primarily by the behavior inthe crystalline portion 214. The heavily doped, polycrystallinesemiconductor regions may have a concentration of dopants (atoms/cm³),that is more than 10 times the concentration of dopants in the lightlydoped, single crystal semiconductor regions, and more preferably 100 to1000 times greater. For example, the heavily doped semiconductor regionsaround 10⁺¹⁷ to 10⁺¹⁹/cm³ doping concentration on the order of, and thelightly doped semiconductor regions have a doping concentration on theorder of 10⁺¹⁴/cm³ to 10⁺¹⁶/cm³.

FIG. 2C also shows that the crystalline region 214 is a singlecrystalline body with an interface for forming the pn-junction on top,and integral with a more heavily doped access line 212 as illustratedmore clearly in FIGS. 2A and 2B. The top of the crystalline region 214includes body protruding above a surface of the single crystalline bodyadjacent the access device by an amount greater than the depth of thedepletion region, that is greater than W_(P) in the embodiment with ap-type crystalline material. Thus, the depletion region that forms incrystalline region 214 is isolated from adjacent diodes by trenches 225in the single crystalline body, having depths greater than the width ofthe depletion region in the crystalline material. This isolates thejunctions from adjacent junctions that can be packed close together. Thetrenches can be formed in a manner self-aligned with the polycrystallineregions by over-etching when patterning the polysilicon regions 216, orusing a sidewall spacer on the polysilicon regions as a etch mask, asshown in FIG. 18B. Of course, other techniques can be applied to form apattern in the surface of the single crystalline body used for theaccess line and the single crystal element of the junction.

Therefore, the low leakage current provided in single crystal junctionsis achieved, or substantially achieved by the structure describedherein. However, the ease of manufacturing provided by forming apolycrystalline more highly doped region 216, is achieved withoutsubstantial costs in the form of leakage current.

Referring to FIGS. 2A and 2B, the memory cell 115 includes a first dopedsemiconductor region 213 having a first conductivity type and a seconddoped semiconductor region 216 on the first doped semiconductor region213, the second doped semiconductor region 216 having a secondconductivity type opposite the first conductivity type. The first dopedsemiconductor region 213 includes a conductively doped region 212overlain by a lightly doped region 214. A pn-junction 215 is definedbetween the lightly doped region 214 of the first doped semiconductorregion 213 and the second doped semiconductor region 216. In the exampleshown in the figures, the first doped semiconductor region is a p-typesemiconductor; the conductively doped region is marked “P+”, and thelightly doped region is marked “P−”. And, in the example shown in thefigures, the second doped semiconductor region is a more heavily-dopedn-type semiconductor, marked “N+”.

The first doped semiconductor region 213 is formed by doping the(single-crystal) semiconductor substrate and, accordingly, the firstdoped semiconductor region is a single-crystal semiconductor. The seconddoped semiconductor region is a doped deposited polysilicon material.Accordingly, the diode is made up of first and second semiconductorregions, defining a pn-junction between them; the first semiconductorregion is formed of a single-crystal semiconductor, and the secondsemiconductor region is formed of a polycrystalline semiconductor.

The doped single-crystal semiconductor regions may be formed in thewafer itself. Alternatively, the doped single-crystal semiconductorregions may be formed in a silicon-on-insulator “SOI” substrate (such asa silicon-insulator-silicon) substrate.

The memory cell 115 includes a conductive cap 218 on the second dopedsemiconductor region 216. The first and second doped semiconductorregions 213, 216 and the conductive cap 218 constitute a multi-layerstack defining diode 121. In the illustrated embodiment the conductivecap 218 includes a silicide containing, for example, Ti, W, Co, Ni, orTa formed using a self-aligned silicide process. Patterned silicideprocesses may be used as well, employing typically a tungsten silicide.The conductive cap 218 assists in maintaining the uniformity of anelectric field impressed across the first and second doped semiconductorregions 213, 216 during operation by providing a contact surface that ismore highly conductive than the semiconductor material of the first andsecond doped semiconductor regions 213, 216. The conductive cap 218 alsoprovides a low resistance ohmic contact between the diode 121 and theoverlying memory element 160. Additionally, the conductive cap 218 canserve as a protective etch stop layer for the second doped semiconductorregion 216 during the manufacture of the memory cell array 100. Inaddition, the silicide used to form the conductive cap is formed inregions 218 a, 218 b on the surface of the more heavily doped region 212and between the memory cells. The conductive cap in regions 218 a, 218 bremoves minority carriers from the more heavily doped region, andimproves the conductivity of the word line formed by the region 212.Also, the conductive cap in the regions 218 a, 218 b provides a lowresistance ohmic contact between conductive plugs 224 and the region212.

In the example shown in FIGS. 2A and 2B, the width of the lightly dopedsingle crystal, semiconductor region 214 beneath the more heavily dopedpolycrystalline semiconductor region 212 is greater than the width ofthe second doped semiconductor region 216, as defined by spacers 250formed adjacent the second doped semiconductor region 216. Conductiveplugs 224, typically tungsten and/or other metals, contact throughcontact openings the more heavily doped semiconductor region 212situated to the side of the second doped semiconductor region 216, andextend upward to make contact with overlying structures, as shown anddescribed below. In alternative embodiments, the conductive plugs neednot be implemented between every cell as shown in FIG. 2B. Instead, someembodiments may provide such plugs in a less dense arrangement, such asevery other cell, every fourth cell, every eighth cell and so ondepending on the implementation of the overlying structures and otherconsiderations.

Optionally, a thin barrier layer (not shown) can be situated at thepn-junction, that is, between the lightly-doped semiconductor region 214and the more heavily-doped semiconductor region 216. The barrier layercan inhibit diffusion of dopants across the pn-junction, and can enhanceperformance of the diode. A suitable barrier layer may be, for example,silicon dioxide (SiO₂) or a silicon oxynitride (SiN_(x)O_(y)); and itmay have a thickness in a range of about 5 to 25 Angstroms, for exampleabout 10 Angstroms.

Arrow 219 in FIG. 2B shows the current flow direction from an overlyingmemory element (not shown in this FIG.) across the pn-junction 215through the diode and on to and through the contact via, and eventuallyto an overlying access line (not shown in this FIG.). The more heavilydoped semiconductor region 212 performs an in-substrate word linefunction of limited length. As mentioned above, the length betweencontacts to the in-substrate, local word line can be extended inembodiments utilizing silicided regions 218 a, 218 b at the surface. Asthe FIG. illustrates, because the width of the lightly dopedsemiconductor region 214 is greater than the width of the second dopedsemiconductor region 216, the current necessarily passes from the seconddoped semiconductor region 216 through the lightly doped semiconductorregion 214 beneath the spacer before it passes on to the more heavilydoped semiconductor region 212.

As is discussed in more detail below, access diodes having anisland-type second region (as shown generally in FIGS. 2A and 2B) may asa result of selected processing steps in various embodiments have thesecond region self-aligned with the first diode region strip 313 definedby the isolation trench 310 (FIG. 3), or not aligned with first dioderegion strip 413 defined by the isolation trench 410 (FIG. 4).

Referring to the self-aligned configuration in FIG. 3, the memory cellincludes a first doped semiconductor region 313 having a firstconductivity type and a second doped semiconductor region 316 on thefirst doped semiconductor region 313, the second doped semiconductorregion 316 having a second conductivity type opposite the firstconductivity type. The first doped semiconductor region 313 includes amore heavily doped region 312 overlain by a lightly doped region 314. Apn-junction 315 is defined between the lightly doped region 314 of thefirst doped semiconductor region 313 and the second doped semiconductorregion 316. Where, as here, the second doped semiconductor region 316 isself-aligned, the width of the second doped semiconductor region is thesame as the width of the lightly doped semiconductor region 314 at thepn-junction 315.

Referring to the non-self-aligned configuration in FIG. 4, the memorycell includes a first doped semiconductor region 413 having a firstconductivity type and a second doped semiconductor region 416 on thefirst doped semiconductor region 413, the second doped semiconductorregion 416 having a second conductivity type opposite the firstconductivity type. The first doped semiconductor region 413 includes amore heavily doped region 412 overlain by a lightly doped region 414. Apn-junction 415 is defined between the lightly doped region 414 of thefirst doped semiconductor region 413 and the second doped semiconductorregion 416. Because, in this configuration, the second dopedsemiconductor region 416 is not self-aligned, the width of the seconddoped semiconductor region is greater than the width of the lightlydoped semiconductor region 414 at the pn-junction 415.

FIGS. 5A, 5B, 5C; 6A, 6B; 7A, 7B; 8A, 8B; and 9A, 9B show examples ofvarious embodiments of memory cells in which various memory elementconfigurations are formed over access diodes having an island-typesecond region that is aligned with the first diode region strip.

Referring to FIGS. 5A, 5B, a conductive plug 320 is formed in contactwith the cap 318, raising the level of the overlying array of memoryelements. A dielectric layer 510 supports the memory element array. Inthis example the memory element 160 includes a bottom electrode inelectrical contact with the second region of the diode, a memorymaterial in contact with the bottom electrode, and a top electrode overthe memory material and electrically coupled with overlying access line(bit line) 120 b. In this configuration the bottom electrode 532 isformed in and extends through a pore in the dielectric layer 510. Thebottom electrode 532 contacts the underlying cap 318 and contacts anoverlying island of a memory material 530 formed over the dielectriclayer 510, and each memory material island is overlain by a topelectrode 534. The top electrode is coupled to access line 120 b by wayof conductive plug 522. In each cell a small area of the phase changematerial contacts the bottom electrode 532, and an active region 533adjacent the contact with the bottom electrode is the region of thememory element 530 in which the memory material is induced to changebetween at least two solid phases.

The pore in the dielectric layer 510 can be formed by, for example, a“keyhole” technique such as the methods, materials, and processes asdisclosed in U.S. patent application Ser. No. 11/855,979 filed on 14Sep. 2007 entitled “Phase Change Memory Cell in Via Array withSelf-Aligned, Self-Converged Bottom Electrode and Method forManufacturing”, which is incorporated by reference herein. For example,the dielectric 510 can be formed on the top surface of access circuitryfollowed by sequentially forming an isolation layer and a sacrificiallayer. Next, a mask having openings close to or equal to the minimumfeature size of the process used to create the mask is formed on thesacrificial layer, the openings overlying the location of the plug 320or diode cap 318. The isolation layer and the sacrificial layers arethen selectively etched using the mask, thereby forming a contactopening in the isolation and sacrificial layers and exposing a topsurface of the dielectric layer 510. After removal of the mask, aselective undercutting etch is performed on the contact opening suchthat the isolation layer is etched while leaving the sacrificial layerand the dielectric layer 510 intact. A fill material is then formed inthe contact opening, which due to the selective undercutting etchprocess results in a self-aligned void in the fill material being formedwithin the contact opening. Next, an anisotropic etching process isperformed on the fill material to open the void, and etching continuesuntil the dielectric layer 510 is exposed in the region below the void,thereby forming a sidewall spacer comprising fill material within thecontact opening. The sidewall spacer has an opening dimensionsubstantially determined by the dimensions of the void, and thus can beless than the minimum feature size of a lithographic process. Next, thedielectric layer 510 is etched using the sidewall spacers as an etchmask, thereby forming an opening in the dielectric layer 510 having adiameter less than the minimum feature size. Next, an electrode layer isformed within the openings in the dielectric layer 510. A planarizingprocess, such as chemical mechanical polishing CMP, is then performed toremove the isolation layer and the sacrificial layer and to form thefirst, or bottom, electrode 532, resulting in the structure illustrated.

Referring particularly to FIG. 5B, the first heavily-doped region 312(doped heavily enough to act as a conductor, with silicided surfaceregions) is electrically coupled to overlying word line 130 b byconductive plugs 224, 526. In the embodiment illustrated in FIG. 5B,conductive plug 320 and conductive plugs 224, 526 include tungsten.Other conductive materials may be used.

FIG. 5C illustrates an alternative embodiment, in which the word line130 b is coupled to the doped regions 312 with a silicide surface, onlyperiodically or only at the periphery of the array. Thus, the conductiveplugs 224, 526 shown in FIG. 5B, are eliminated in the embodiment shownin FIG. 5C. Otherwise, FIG. 5C is the same as FIG. 5B.

The bottom electrode 532 may include, for example, TiN or TaN. TiN maybe preferred in embodiments in which memory element 530 comprises GST(discussed below) because it makes good contact with GST, it is a commonmaterial used in semiconductor manufacturing, and it provides a gooddiffusion barrier at the higher temperatures at which GST transitions,typically in the 600-700° C. range. Alternatively, the bottom electrode532 may be TiAlN or TaAlN, or comprises, for further examples, one ormore elements selected from the group consisting of Ti, W, Mo, Al, Ta,Cu, Pt, Ir, La, Ni, N, O, and Ru and combinations thereof.

In the illustrated embodiment dielectric 510 surrounding the bottomelectrode and underlying the memory element includes silicon nitride.The dielectric material for the layer 510 may be selected to facilitatepore formation by selective etch through an opening in an overlyingtemporary material, such as a silicon oxide, for example.

The top electrode 534 and the bit lines 120 may include, for example,any of the materials described above with reference to the bottomelectrode 532.

The dielectric material constituting the fill in the isolation trenchmay include, for example, silicon oxide, silicon dioxide, and any othermaterial sufficient to electrically isolate the diode ridges.

In the illustrated example the memory element 530 includes a phasechange material. The memory element 530 may comprise, for example, oneor more materials from the group of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn,Cu, Pd, Pb, Ag, S, Si, O, P, As, N and Au.

Embodiments of the memory cells described herein include phase changebased memory materials, including chalcogenide based materials and othermaterials, for the memory element. Chalcogens include any of the fourelements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te),forming part of group VIA of the periodic table. Chalcogenides comprisecompounds of a chalcogen with a more electropositive element or radical.Chalcogenide alloys comprise combinations of chalcogenides with othermaterials such as transition metals. A chalcogenide alloy usuallycontains one or more elements from group IVA of the periodic table ofelements, such as germanium (Ge) and tin (Sn). Often, chalcogenidealloys include combinations including one or more of antimony (Sb),gallium (Ga), indium (In), and silver (Ag). Many phase change basedmemory materials have been described in technical literature, includingalloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te,Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te andTe/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloycompositions may be workable. The compositions can be characterized asTe_(a)Ge_(b)Sb_(100−(a+b)). One researcher has described the most usefulalloys as having an average concentration of Te in the depositedmaterials well below 70%, typically below about 60% and ranged ingeneral from as low as about 23% up to about 58% Te and most preferablyabout 48% to 58% Te. Concentrations of Ge were above about 5% and rangedfrom a low of about 8% to about 30% average in the material, remaininggenerally below 50%. Most preferably, concentrations of Ge ranged fromabout 8% to about 40%. The remainder of the principal constituentelements in this composition was Sb. These percentages are atomicpercentages that total 100% of the atoms of the constituent elements.(Ovshinsky U.S. Pat. No. 5,687,112, cols. 10-11.) Particular alloysevaluated by another researcher include Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7(Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks forHigh-Data-Rate Recording”, SPIE v. 3109, pp. 28-37 (1997).) Moregenerally, a transition metal such as chromium (Cr), iron (Fe), nickel(Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloysthereof may be combined with Ge/Sb/Te to form a phase change alloy thathas programmable resistive properties. Specific examples of memorymaterials that may be useful are given in Ovshinsky '112 at columns11-13, which examples are hereby incorporated by reference.

Chalcogenides and other phase change materials are doped with impuritiesin some embodiments to modify conductivity, transition temperature,melting temperature, and other properties of memory elements using thedoped chalcogenides. Representative impurities used for dopingchalcogenides include nitrogen, silicon, oxygen, silicon dioxide,silicon nitride, copper, silver, gold, aluminum, aluminum oxide,tantalum, tantalum oxide, tantalum nitride, titanium and titanium oxide.See, e.g., U.S. Pat. No. 6,800,504, and U.S. Patent ApplicationPublication No. 2005/0029502.

Phase change alloys are capable of being switched between a firststructural state in which the material is in a generally amorphous solidphase, and a second structural state in which the material is in agenerally crystalline solid phase in its local order in the activechannel region of the cell. These alloys are at least bistable. The termamorphous is used to refer to a relatively less ordered structure, moredisordered than a single crystal, which has the detectablecharacteristics such as higher electrical resistivity than thecrystalline phase. The term crystalline is used to refer to a relativelymore ordered structure, more ordered than in an amorphous structure,which has detectable characteristics such as lower electricalresistivity than the amorphous phase. Typically, phase change materialsmay be electrically switched between different detectable states oflocal order across the spectrum between completely amorphous andcompletely crystalline states. Other material characteristics affectedby the change between amorphous and crystalline phases include atomicorder, free electron density and activation energy. The material may beswitched either into different solid phases or into mixtures of two ormore solid phases, providing a gray scale between completely amorphousand completely crystalline states. The electrical properties in thematerial may vary accordingly.

Phase change alloys can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase change material to agenerally amorphous state. A longer, lower amplitude pulse tends tochange the phase change material to a generally crystalline state. Theenergy in a shorter, higher amplitude pulse is high enough to allow forbonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined, without undue experimentation,specifically adapted to a particular phase change alloy. In followingsections of the disclosure, the phase change material is referred to asGST, and it will be understood that other types of phase changematerials can be used. A material useful for implementation of a PCRAMdescribed herein is Ge₂Sb₂Te₅.

Other programmable resistance memory materials may be used in otherembodiments of the invention, including other materials that usedifferent crystal phase changes to determine resistance, or other memorymaterials that use an electrical pulse to change the resistance state.Examples include materials for use in resistance random access memory(RRAM) such as metal-oxides including tungsten-oxide (WO_(x)), NiO,Nb₂O₅, CuO₂, Ta₂O₅, Al₂O₃, CoO, Fe₂O₃, HfO₂, TiO₂, SrTiO₃, SrZrO3,(BaSr)TiO₃. Additional examples include materials for use inmagnetoresistance random access memory (MRAM), for example at least oneof CoFeB, Fe, Co, Ni, Gd, Dy, CoFe, NiFe, MnAs, MnBi, MnSb, CrO₂,MnOFe₂O₃, FeOFe₂O₅, NiOFe₂O₃, MgOFe₂, EuO, and Y₃Fe₅O₁₂. See, forexample, US Publication No 2007/0176251 entitled “Magnetic Memory Deviceand Method of Fabricating the Same”, which is incorporated by referenceherein. Additional examples include solid electrolyte materials used forprogrammable-metallization-cell (PMC) memory, or nano-ionic memory, suchas silver-doped germanium sulfide electrolytes and copper-dopedgermanium sulfide electrolytes. See, for example, N. E. Gilbert et al.,“A macro model of programmable metallization cell devices,” Solid-StateElectronics 49 (2005) 1813-1819, which is incorporated by referenceherein.

An exemplary method for forming chalcogenide material usesPVD-sputtering or magnetron-sputtering method with source gas(es) of Ar,N₂, and/or He, etc. at the pressure of 1 mTorr˜100 mTorr. The depositionis usually done at room temperature. A collimator with an aspect ratioof 1˜5 can be used to improve the fill-in performance. To improve thefill-in performance, a DC bias of several tens of volts to severalhundreds of volts is also used. On the other hand, the combination of DCbias and the collimator can be used simultaneously.

A post-deposition annealing treatment in a vacuum or in an N₂ ambient isoptionally performed to improve the crystallize state of chalcogenidematerial. The annealing temperature typically ranges from 100° C. to400° C. with an anneal time of less than 30 minutes.

Alternatively, the chalcogenide material may be formed by chemical vapordeposition (CVD).

FIGS. 6A, 6B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is aligned with the first diode regionstrip. The construction is similar to that of FIGS. 5A, 5B, except thathere no conductive plug separates the cap 318 from the bottom electrode532 and the dielectric layer 510 surrounding the bottom electrode andunderlying the memory element 530. As in FIGS. 5A, 5B, the bottomelectrode 532 is formed in and extends through a pore in the dielectriclayer 510. The bottom electrode 532 contacts the underlying cap 318 andcontacts an overlying island of a memory material 530 formed over thedielectric layer 510, and each memory material island is overlain by atop electrode 534. The top electrode is coupled to access line 120 b byway of conductive plug 522. In each cell a small area of the phasechange material contacts the bottom electrode 532, and an active region533 adjacent the contact with the bottom electrode is the region of thememory element 530 in which the memory material is induced to changebetween at least two solid phases.

Referring particularly to FIG. 6B, the first heavily-doped region 312 iselectrically coupled to overlying word line 130 b at locations not shownin local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 7A, 7B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is aligned with the first diode regionstrip. The construction is similar to that of FIGS. 6A, 6B, except thathere the memory material is formed as strips 730 rather than islands;and the top electrode, which is also formed as a strip 734 overlying thememory material, serves as a bit line. As in the example of FIGS. 6A,6B, the bottom electrode 532 is formed in and extends through a pore inthe dielectric layer 510. The bottom electrode 532 contacts theunderlying cap 318 and contacts an overlying strip of a memory material730 formed over the dielectric layer 510, and each memory material stripis overlain by a top electrode strip 734. In each cell a small area ofthe phase change material contacts the bottom electrode 532, and anactive region 533 adjacent the contact with the bottom electrode is theregion of the memory strip 730 in which the memory material is inducedto change between at least two solid phases.

Referring particularly to FIG. 7B, the first heavily-doped region 312 iselectrically coupled to overlying word line 130 b at locations not shownin local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 8A, 8B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is aligned with first diode region strip.In this example the memory material is formed as strips 830 rather thanislands; and the top electrode, which is also formed as a strip 834overlying the memory material, serves as a bit line. Moreover, in thisexample there are pores in the dielectric layer 810, but there is noseparate bottom electrode. Instead, in this example a portion of thememory material extends from the strip 830 through the pores, as shownat 832, and makes contact with the cap 318 at the top of the diode stack121.

Referring particularly to FIG. 8B, the first heavily-doped region 312 iselectrically coupled to overlying word line 130 b at locations not shownin local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 9A, 9B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is aligned with the first diode regionstrip. In this example there are pores in the dielectric layer 910 and,as in the example shown in FIGS. 8A, 8B there is no separate bottomelectrode. But here there are no strips of memory material, and the topelectrode 934, which also serves as the bit line 930, overlies thedielectric 910 directly. The memory element for each cell is formedsolely in the pores, as shown at 932, and makes contact below with thecap 318 at the top of the diode stack 121 and above with the overlyingtop electrode 934.

Referring particularly to FIG. 9B, the first heavily-doped region 312 iselectrically coupled to overlying word line 130 b at locations not shownin local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 10A, 10B; 11A, 11B; and 12A, 12B show examples of variousembodiments of memory cells in which the access diodes have anisland-type second region that is not aligned with the first dioderegion strip.

FIGS. 10A, 10B illustrate an embodiment of a memory cell array in whichmemory elements are formed over access diodes having an island-typesecond region that is not aligned with the first diode region strip. Theconstruction is similar to that of FIGS. 8A, 8B, except that here thedielectric fill 1010 upon which the memory material strip 1030 is formedalso surrounds at least the upper island part of the diode stack 121,including the second doped semiconductor 416 and the cap 418. As in theexample of FIGS. 8A, 8B, there is no separate bottom electrode. Instead,in this example there are pores in the dielectric fill 1010, but aportion of the memory material extends from the strip 1030 through thepores, as shown at 1032, and makes contact with the cap 418 at the topof the diode stack 121.

Referring particularly to FIG. 10B, the first heavily-doped region 412is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 11A, 11B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is not aligned with the first dioderegion strip. In this example, a dielectric fill 1110 surrounds at leastthe upper island part of the diode stack 121, including the second dopedsemiconductor 416 and the cap 418. The bottom electrode 1132 is formedin and extends through a pore in the dielectric layer 1110. The bottomelectrode 1132 contacts the underlying cap 418 and contacts an overlyingmemory material island 1130 formed over the dielectric layer 1110, andthe row of memory material islands is overlain by a top electrode strip1134, which serves as a bit line. In each cell a small area of the phasechange material contacts the bottom electrode 1132, and an active region1133 adjacent the contact with the bottom electrode is the region of thememory material island 1130 in which the memory material is induced tochange between at least two solid phases.

Referring particularly to FIG. 11B, the first heavily-doped region 412is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 12A, 12B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having anisland-type second region that is overlies the first diode region strip.In this example, as in the example of FIGS. 11A, 11B, a dielectric fill1210 surrounds at least the upper island part of the diode stack 121,including the second doped semiconductor 416 and the cap 418, and thememory material is formed as islands 1230 rather than a strips; and thetop electrode, which is also formed as a strip 1234 overlying the memorymaterial, serves as a bit line. However, in this example there are poresin the dielectric layer 1210, but there is no separate bottom electrode.Instead, in this example a portion of the memory material extends fromthe island 1230 through the pores, as shown at 1232, and makes contactwith the cap 418 at the top of the diode stack 121.

Referring particularly to FIG. 12B, the first heavily-doped region 412is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 312 by conductive plugs not shown.Alternatively, the conductive plugs can be placed between each cell asshown in FIG. 5B.

FIGS. 13-20C show stages in an embodiment of a process for making accessdiodes having an island-type second region that is self-aligned with thefirst diode region strip.

A semiconductor substrate is provided (typically in the form of asemiconductor wafer, for example a silicon wafer). Where, as in thisexample, the substrate is a P-type semiconductor, an N-well 1310 isformed, and then the wafer is doped to provide a relativelyheavily-doped region overlain by a relatively lightly-doped region ofthe same conductivity type (P-type in this example). The result is shownin FIG. 13. In the figures, the relatively heavily-doped region 1312 isindicated by “P+” and the relatively lightly-doped region 1314 isindicated by “P−”

Thereafter, a layer 1316 of polycrystalline semiconductor material isformed over the lightly-doped region 1314. The polycrystallinesemiconductor material (typically polysilicon) is relativelyheavily-doped compared to the doping concentration of the lightly-dopedregion, and has a conductivity type opposite that of the underlyinglightly-doped region of the substrate. The result is shown in FIG. 14.In the figures, the relatively heavily-doped polycrystalline layer 1316is indicated by “N+”.

Where, optionally, a barrier layer is desired at the pn-junction, thelayer can be formed by growing or depositing a dielectric layer on theexposed lightly-doped silicon region 1314 prior to forming the overlyinglayer 1316 of polysilicon. A suitable barrier layer may be, for example,silicon dioxide (SiO₂) or a silicon oxynitride (SiN_(x)O_(y)); and itmay be formed to a thickness in a range of about 5 to 25 Angstroms, forexample about 10 Angstroms.

Thereafter, an isolation trench is formed, resulting in ridges separatedby a dielectric 1530, as shown in FIGS. 15A, 15B, 15C. Each ridgeincludes a strip of the relatively heavily doped crystallinesemiconductor material (1312, P+) overlying the N-well, overlain by astrip of the relatively lightly doped crystalline semiconductor material(1314, P−). These together constitute a first region 1313, which isoverlain by a strip of the relatively heavily doped (opposite type)polycrystalline material (1316, N+).

Thereafter, a mask is formed over the ridges and the dielectric 1530between the ridges, as shown in FIG. 16. The mask is patterned as strips1612 traversing the strips 1316 of the relatively heavily dopedpolycrystalline material.

Thereafter, an etch is carried out to remove any exposed relativelyheavily doped polycrystalline material (and exposed dielectricmaterial), and the mask is removed, with a result as shown in FIGS. 17A,17B, 17C. The etch stops at the surface of the relatively lightly-dopedcrystalline semiconductor material (1314, P−), so that the resultingconstruct consists of islands 1716 of relatively heavily dopedpolycrystalline material upon the first region 1313 ridges. Inembodiments in which the spacing between N+ regions 1716 is small, theprocess includes overetching so that the etch continues past the surfaceof the lightly-doped crystalline semiconductor material 1314, to a depthgreater than the depth of the pn-junction (i.e. greater than width W_(P)of the depletion region as shown in FIG. 2C), to isolate the adjacentdepletion regions. This overetching is shown in FIG. 18B, but can beapplied in any embodiment including closely packed cells, includingembodiments in which other alignment processes are used.

Thereafter, spacers 1810 are formed at the sides of the islands 1716 ofrelatively heavily doped polycrystalline material, as shown in FIGS.18A, 18B, 18C. The spacers 1810 mask narrow regions 1820 of therelatively lightly-doped crystalline semiconductor material (1314, P−)adjacent each island 1716, and the width of the masked region can bedetermined by the width of the spacers 1810. This leaves regions of therelatively lightly-doped crystalline semiconductor material exposed, asindicated at 1816.

Thereafter, an implant is performed to heavily dope the exposed regionsof the relatively lightly-doped crystalline semiconductor material, witha result as shown in FIGS. 19A, 19B, 19C. The regions 1512 beneath theexposed surfaces 1816 of the ridge 1313 of crystalline semiconductormaterial are now heavily doped, with the same conductivity type as therelatively heavily doped crystalline semiconductor material (1312, P+)overlying the N-well.

Thereafter, an electrically conductive cap 2018, such as a silicide asdescribed above is formed over the islands 1716 of relatively heavilydoped polycrystalline material, and over the relatively heavily dopedcrystalline material 1512, with the result shown in FIGS. 20A, 20B, 20C.The resulting diode structure (refer also to FIGS. 2A, 2B, 3) is nowready for formation of overlying memory elements, as described in detailbelow.

FIGS. 21A-25C show stages in an embodiment of a process for makingaccess diodes having an island-type second region that is notself-aligned with the first diode region strip.

In these examples as for the examples discussed with reference to FIGS.13-20C, a semiconductor substrate is provided (typically in the form ofa semiconductor wafer, for example a silicon wafer). Where, as in thisexample, the substrate is a P-type semiconductor, an N-well 2110 isformed in the array region, and N-well and P-wells are formed in partsof the peripheral regions. In FIG. 21A, a peripheral active region 2129is shown in a N-well. Then the wafer is doped in the array region toprovide a relatively heavily-doped region overlain by a relativelylightly-doped region of the same conductivity type (P-type in thisexample). In the figures, the relatively heavily-doped region 2112 isindicated by “P+” and the relatively lightly-doped region 2114 isindicated by “P−”. Isolation trenches are formed in both the array andperipheral regions, resulting in ridges separated by a dielectric 2130,as shown in FIGS. 21A, 21B, 21C, and in the active regions, such asregions 2129 in the peripheral regions shown with arbitrary shapes inFIG. 21C, whereas it will be understood that the transistors in theperipheral regions are used for logic and other purposes on the deviceand will have complex layouts. Each ridge in the array includes a stripof the relatively heavily doped crystalline semiconductor material(2112, P+) overlying the N-well, overlain by a strip of the relativelylightly doped crystalline semiconductor material (2114, P−). Thesetogether constitute a first diode region 2113, formed entirely of dopedcrystalline semiconductor material. Although not shown in the drawings,a sidewall of silicide can be formed to improve the conductivity of theconductor formed by the strips of p-type material in the ridges. Thiscan be done in embodiments like that shown in FIG. 21A, by etching backa portion of the fill 2130 to expose the sides of the regions 2114 and2112. Then a silicide precursor is deposited on the exposed sides andannealed to cause silicide formation. Then the remaining silicideprecursor on the substrate is removed leaving the self-aligned silicideelements on the sides of the ridges. Typical silicide precursors includemetals or combinations of metals such as cobalt, titanium, nickel,molybdenum, tungsten, tantalum, and platinum. Also, silicide precursorsmay include metal nitrides or other metal compounds. The resultingsilicide strip, not shown, removes minority carriers from the p-typematerial and improves conductivity of the structures.

Thereafter, gate dielectric layer 2128 is deposited in the peripheralregion, either by a blanket process followed by removal in the arrayarea, or by a process which masks the array area. Then a layer ofpolycrystalline semiconductor material is formed over both theperipheral regions and the array regions, including over thelightly-doped region 2114 and the surface of the isolation trenchdielectric 2130. The polycrystalline semiconductor material (typicallypolysilicon) is relatively heavily-doped, and has a conductivity typeopposite that of the underlying lightly-doped region of the substrate.Then a cap layer is formed on the polycrystalline semiconductor layer,and the layers are patterned to form strips traversing the strips 2114of the relatively lightly doped crystalline semiconductor material(2114, P−) and to form appropriate interconnect and gate structures inthe peripheral regions. The result is shown in FIGS. 22A, 22B, 22C. Inthe figures, each strip 2116 of the relatively heavily-dopedpolycrystalline material is indicated by “N+”; and is overlain by astrip 2118 of cap material. In this process, and similar processes, asingle polysilicon process can be shared for both the heavily dopedpolysilicon elements in the array and peripheral polysilicon structures,saving substantial costs in manufacturing. In this embodiment,transistor gate structures in the peripheral circuitry region of thedevice and the heavily doped polysilicon elements of the diodes compriserespective features in a single polysilicon layer. Thereafter aninterlayer dielectric can formed, and openings are formed in both theperipheral device region and the memory array region. The openings arefilled with a conductive contact plug material such as tungsten in theperipheral device region, and depending on the memory cell embodiment tobe implemented, with elements completing the memory cells in the arrayregion. Optionally, and preferably in some embodiments, a silicide layer(not shown) may be formed on the surface of layer 2412 as well (See,FIG. 24B).

Where, optionally, a barrier layer is desired at the pn-junction, thelayer can be formed by growing or depositing a dielectric layer on theexposed lightly-doped silicon region 2114 prior to forming the overlyinglayer 2116 of polysilicon. A suitable barrier layer may be, for example,silicon dioxide (SiO2) or a silicon oxynitride (SiNxOy); and it may beformed to a thickness in a range of about 5 to 25 Angstroms, for exampleabout 10 Angstroms.

Thereafter, focusing on the array area, spacers 2310 are formed at thesides of the strips 2116 of relatively heavily doped polycrystallinematerial. The spacers 2310 mask narrow regions 2320 of the relativelylightly-doped crystalline semiconductor material (2114, P−) adjacenteach strip 2116, and the width of the masked region can be determined bythe width of the spacers 2310. This leaves regions of the relativelylightly-doped crystalline semiconductor material exposed, as indicatedat 2316.

Thereafter, an implant is performed to heavily dope the exposed regionsof the crystalline semiconductor material, with a result as shown inFIGS. 24A, 24B, 24C. The regions 2412 beneath the exposed surfaces 2316of the ridge 2113 of crystalline semiconductor material are now heavilydoped, with the same conductivity type as the relatively heavily dopedcrystalline semiconductor material (2112, P+) overlying the N-well.

Thereafter, a mask is applied over the strips of cap material 2118 andpolycrystalline material 2116, and the dielectric material 2130 betweenthe ridges, and is patterned to cover at least portions of the capmaterial 2118 and polycrystalline material 2116 generally overlying theridges 2113 of crystalline semiconductor material. An etch operation isperformed to remove any exposed relatively heavily doped polycrystallinematerial, and the mask is removed, with a result as shown in FIGS. 25A,25B, 25C. The etch stops at the surface of the implanted crystallinesemiconductor material (2412, P+), so that the resulting constructconsists of islands 2520 of relatively heavily doped polycrystallinematerial 2516 overlain by cap 2518 upon the first region 2113 ridges.Optionally, and preferably in some embodiments, a silicide layer (notshown) may be formed on the surface of layer 2412 as well. The resultingdiode structure (refer also to FIGS. 2A, 2B, 4) is now ready forformation of overlying memory elements, as described in detail below.

FIGS. 26A and 26B show a portion of an embodiment of a memory cellarray, in which the second region of the diode has the form of a pillar,in diagrammatic sectional views. FIG. 26A is taken along a bit line 120,and FIG. 26B is taken along a word line 130.

Referring to FIGS. 26A and 26B, the memory cell 115 includes a firstdoped semiconductor region 2613 having a first conductivity type and asecond doped semiconductor plug 2616 on the first doped semiconductorregion 2613, the second doped semiconductor plug 2616 having a secondconductivity type opposite the first conductivity type. The first dopedsemiconductor region 2613 includes a more heavily doped region 2612overlain by a lightly doped region 2614. A pn-junction 2615 is definedbetween the lightly doped region 2614 of the first doped semiconductorregion 2613 and the second doped semiconductor plug 2616. In the exampleshown in the figures, the first doped semiconductor region is a p-typesemiconductor; the conductively doped region is marked “P+”, and thelightly doped region is marked “P−”. And, in the example shown in thefigures, the second doped semiconductor region is a more heavily-dopedn-type semiconductor, marked “N+”, having a doping concentration higherthan that of the lightly-doped region.

The first doped semiconductor region 2613 is formed by doping the(single-crystal) semiconductor substrate and, accordingly, the firstdoped semiconductor region is a single-crystal semiconductor. The seconddoped semiconductor region is a doped deposited polysilicon plug, formedwithin a via through an insulating layer (not shown). Accordingly, thediode is made up of first and second semiconductor regions, defining apn-junction between them; the first semiconductor region is formed of asingle-crystal semiconductor, and the second semiconductor region isformed of a polycrystalline semiconductor.

The doped single-crystal semiconductor regions may be formed in thewafer itself. Alternatively, the doped single-crystal semiconductorregions may be formed in a silicon-on-insulator “SOI” substrate (such asa silicon-insulator-silicon) substrate.

The memory cell 115 includes a conductive cap 2618 on the second dopedsemiconductor plug 2616. The first and second doped semiconductorregions 2613, 2616 and the conductive cap 2618 constitute a multi-layerstack defining diode 121. In the illustrated embodiment the conductivecap 2618 includes a silicide containing, for example, Ti, W, Co, Ni, orTa. The conductive cap 2618 assists in maintaining the uniformity of anelectric field impressed across the first and second doped semiconductorregions 2613, 2616 during operation by providing a contact surface thatis more highly conductive than the semiconductor material of the firstand second doped semiconductor regions 2613, 2616. The conductive cap2618 also provides a low resistance ohmic contact between the diode 121and the overlying memory element 160. Additionally, the conductive cap2618 can serve as a protective etch stop layer for the second dopedsemiconductor plug 2616 during the manufacture of the memory cell array100. Optionally, and preferably in some embodiments, a silicide layermay be formed on the surface of layer 2612 as well.

In the example shown in FIGS. 26A and 26B, the width of the lightlydoped semiconductor region 2614 overlying the more heavily dopedsemiconductor region 2612 is greater than the width of the second dopedsemiconductor plug 2616. A conductive plug 2624 contacts the moreheavily doped semiconductor region 2612 in a region of the array spacedaway from or situated to the side of the second doped semiconductor plug2616, and extends upward to make contact with overlying structures, asshown and described below.

Optionally, a barrier layer is desired at the pn-junction, the layer canbe formed by growing or depositing a dielectric layer on the exposedlightly-doped silicon region 2614 prior to forming the overlying layer2616 of polysilicon. A suitable barrier layer may be, for example,silicon dioxide (SiO₂) or a silicon oxynitride (SiN_(x)O_(y)); and itmay be formed to a thickness in a range of about 5 to 25 Angstroms, forexample about 10 Angstroms.

Arrow 2619 in FIG. 26B shows the current flow direction from anoverlying memory element (not shown in this FIG.) across the pn-junction2615 through the diode and on to and through the contact via 2624, andeventually to an overlying access line (not shown in this FIG.). As theFIG. illustrates, because the lightly doped semiconductor region 2614 isgreater than the width of the second doped semiconductor plug 2616, thecurrent necessarily passes from the second doped semiconductor plug 2616through the lightly doped semiconductor region 2614 beneath the spacerbefore it passes on to the more heavily doped semiconductor region 2612.

FIGS. 27A, 27B; 28A, 28B; 29A, 29B; 30A, 30B; 31A, 31B; 32A, 32B; and33A, 33B show examples of various embodiments of memory cells in whichthe access diodes have a second region in the form of a pillar.

FIGS. 27A, 27B illustrate an embodiment of a memory cell array in whichmemory elements are formed over access diodes having a pillar-typesecond region. In this example a dielectric layer 2710 overlies the caps2618 and adjacent dielectric fill 2630. In this example there is noseparate bottom electrode; a top electrode strip 2730 overlies thedielectric layer 2710 and, in this example a bit line 2734 overlies thetop electrode strip 2730 in direct electrical contact. Pores in thedielectric layer 2710 are narrower at the lower side, where thedielectric layer 2710 overlies the caps 2618, than at the upper side,where the dielectric layer 2710 underlies the top electrode 2730. Thememory element for each cell is formed solely in the pores, as shown at2732, and makes contact below at the narrower end with the cap 2618 atthe top of the diode stack 121 and above at the wider end with theoverlying top electrode 2730.

Referring particularly to FIG. 27B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown.

FIGS. 28A, 28B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having a pillar-typesecond region. In this example a dielectric layer 2810 supports thememory element array. The memory element includes a bottom electrode inelectrical contact with the second region of the diode, a memorymaterial in contact with the bottom electrode, and a top electrode overthe memory material and electrically coupled with overlying access line(bit line) 120 b. In this configuration the bottom electrode 2832 isformed in and extends through a pore in the dielectric layer 2810. Thebottom electrode 2832 contacts the underlying cap 2618 and contacts anoverlying island of a memory material 2830 formed over the dielectriclayer 2810, and each memory material island is overlain by a topelectrode 2834. The top electrode is coupled to access line 120 b by wayof conductive plug 2822. In each cell a small area of the phase changematerial contacts the bottom electrode 2832, and an active regionadjacent the contact with the bottom electrode is the region of thememory element 2830 in which the memory material is induced to changebetween at least two solid phases.

Referring particularly to FIG. 28B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide. FIGS. 29A, 29B illustrate another embodiment of a memorycell array in which memory elements are formed over access diodes havinga pillar-type second region. In this example, as in the example of FIGS.28A, 28B, a dielectric fill 2620 surrounds at least the upper islandpart of the diode stack 121, including the second doped semiconductor2616 and the cap 2618, and the memory material is formed as islands 2930rather than a strips; and a top electrode, which is also formed as astrip 2934 overlies the memory material and is coupled to access line(bit line) 120 b by way of conductive plug 2922. However, in thisexample there are pores in the dielectric layer 2910, but there is noseparate bottom electrode. Instead, in this example a portion of thememory material extends from the island 2930 through the pores, as shownat 2932, and makes contact with the cap 2618 at the top of the diodestack 121.

Referring particularly to FIG. 29B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide.

FIGS. 30A, 30B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having a pillar-typesecond region. In this example there are pores in the dielectric layer3010 and, as in the example shown in FIGS. 29A, 29B there is no separatebottom electrode. But here there are no strips or islands of memorymaterial overlying the dielectric layer, and the top electrode 3034 isin the form of an island and overlies the dielectric 3010 directly. Thememory element for each cell is formed solely in the pores, as shown at3032, and makes contact below with the cap 2618 at the top of the diodestack 121 and above with the overlying top electrode 3034.

Referring particularly to FIG. 30B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide.

FIGS. 31A, 31B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having a pillar-typesecond region. A dielectric layer 3110 supports the memory elementarray. As in FIGS. 28A, 28B, 28C, the memory element includes a bottomelectrode in electrical contact with the second region of the diode, amemory material in contact with the bottom electrode, and a topelectrode over the memory material and electrically coupled withoverlying access line (bit line) 120 b. In this configuration the bottomelectrode 3132 is formed in and extends through a pore in the dielectriclayer 3110. The bottom electrode 3132 contacts the underlying cap 2618and contacts an overlying strip of a memory material 3130 formed overthe dielectric layer 3110, and each memory material strip is overlain bya top electrode strip 3134, which also serves as a bit line 120 b. Ineach cell a small area of the phase change material contacts the bottomelectrode 3132, and an active region 3133 adjacent the contact with thebottom electrode is the region of the memory element 3130 in which thememory material is induced to change between at least two solid phases.

Referring particularly to FIG. 31B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b by at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide. FIGS. 32A, 32B illustrate another embodiment of a memorycell array in which memory elements are formed over access diodes havinga pillar-type second region. In this example the memory material isformed as strips 3230 rather than islands; and the top electrode, whichis also formed as a strip 3234 overlying the memory material, serves asa bit line. Moreover, in this example there are pores in the dielectriclayer 3210, but there is no separate bottom electrode. Instead, in thisexample a portion of the memory material extends from the strip 3230through the pores, as shown at 3232, and makes contact with the cap 2618at the top of the diode stack 121.

Referring particularly to FIG. 32B, the first heavily-doped region 2612is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide.

FIGS. 33A, 33B illustrate another embodiment of a memory cell array inwhich memory elements are formed over access diodes having a pillar-typesecond region. In this example there are pores in the dielectric layer3310 and, as in the example shown in FIGS. 33A, 33B there is no separatebottom electrode. But here there are no strips of memory material, andthe top electrode 3334, which also serves as the bit line 3330, overliesthe dielectric 3310 directly. The memory element for each cell is formedsolely in the pores, as shown at 3332, and makes contact below with thecap 2618 at the top of the diode stack 121 and above with the overlyingtop electrode 3334.

Referring particularly to FIG. 33B, the first heavily-doped region 312is electrically coupled to overlying word line 130 b at locations notshown in local segments of the region 2612 by conductive plugs notshown. Also, the surface of heavily-doped region 2612 can be capped witha silicide.

FIGS. 34A-39C show stages in an embodiment of a process for makingaccess diodes having a second region that has the form of a pillar.

In these examples as for the examples discussed with reference to FIGS.13-20C, a semiconductor substrate is provided (typically in the form ofa semiconductor wafer, for example a silicon wafer). Where, as in thisexample, the substrate is a P-type semiconductor, an N-well 2110 isformed, and then the wafer is doped to provide a relativelyheavily-doped region overlain by a relatively lightly-doped region ofthe same conductivity type (P-type in this example). In the figures, therelatively heavily-doped region 2112 is indicated by “P+” and therelatively lightly-doped region 2114 is indicated by “P−”. An isolationtrench is formed, resulting in ridges separated by a dielectric 2130, asshown in FIGS. 21A, 21B, 21C. Each ridge includes a strip of therelatively heavily doped crystalline semiconductor material (2112, P+)overlying the N-well, overlain by a strip of the relatively lightlydoped crystalline semiconductor material (2114, P−). These togetherconstitute a first diode region 2113, formed entirely of dopedcrystalline semiconductor material.

The doped single-crystal semiconductor regions may be formed in thewafer itself. Alternatively, the doped single-crystal semiconductorregions may be formed in a silicon-on-insulator “SOI” substrate (such asa silicon-insulator-silicon) substrate.

Thereafter, a layer of dielectric 3410 is formed on the structure shownin FIGS. 21A, 21B, 21C, resulting in the structure shown in FIGS. 34A,34B, 34C.

Thereafter, an array of openings 3520 are formed through the dielectric3410 of the structure shown in FIGS. 34A, 34B, 34C to expose areas 3522of the top surfaces of relatively lightly doped crystallinesemiconductor material (2114, P−), resulting in the structureillustrated in FIGS. 35A, 35B, 36. The openings 3520 can be formed by avia etch technique.

Thereafter, doped polysilicon plugs 3716 are formed in the openings 3520of the structure shown in FIGS. 35A, 35B, 36, resulting in the structureshown in FIGS. 37A, 37B, 37C. The doped polysilicon plugs 3716 have aconductivity opposite that of the relatively lightly doped crystallinesemiconductor material (2114, P−), and thus the plugs 3716 contact acorresponding lightly doped crystalline semiconductor material (2114,P−) to define a pn-junction 3715 therebetween. The doped polysiliconplugs 3716 may be formed by, for example, deposition of dopedpolysilicon material on the structure shown in FIGS. 35A, 35B, 36,followed by a planarization process such as Chemical MechanicalPolishing (CMP).

Where, optionally, a barrier layer is desired at the pn-junction, thelayer can be formed by growing or depositing a dielectric layer on theexposed areas 3522 of the lightly-doped silicon layer 2114 prior toforming the polysilicon plugs 3716 in the openings 3520 through thedielectric 3510. A suitable barrier layer may be, for example, silicondioxide (SiO₂) or a silicon oxynitride (SiN_(x)O_(y)); and it may beformed to a thickness in a range about 5-25 Angstroms, for example about10 Angstroms.

Optionally, a second array of openings 3820 are formed through thedielectric 3510 of the structure shown in FIGS. 37A, 37B, 37C to exposeareas 3822 of the top surfaces of implanted relatively highly dopedcrystalline semiconductor material (2414, P+), resulting in thestructure illustrated in FIGS. 38A, 38B, 38C. The openings 3820 can beformed by a via etch technique. Optionally, silicide may be formed ontop of the material 2414. Alternatively, as mentioned above, contactopenings 3820 are used only occasionally in the array, or at theperiphery of the array.

Conductive plugs 3924 are formed by depositing conductive material inthe openings 3820 acting a vias through insulating layer 3510 in thestructure illustrated in FIGS. 38A, 38B, 38C; and conductive caps 3918are formed at the top surface of the doped polysilicon plugs 3716resulting in the structure shown in FIGS. 39A, 39B, 39C. The resultingconstruct consists of pillars of relatively heavily dopedpolycrystalline material (plug 3916) overlain by cap 3918 upon thelightly-doped regions of the first diode region 2413 ridges; andconductive plugs 3924 providing for electrical connection at the diodeconstruct surface to the heavily-doped regions 2412 of the first dioderegion 2413 ridges. The resulting diode structure (refer also to FIGS.26A, 26B) is now ready for formation of overlying memory elements, asdescribed in detail below.

FIGS. 40A-44C show stages in an embodiment of a process for formingmemory elements over access diodes having an island-type second region,consisting of a patterned polysilicon body, that is self-aligned withthe first diode region strip (as in FIGS. 7A, 7B).

Beginning for example with a configuration as shown in FIGS. 20A, 20B,20C, a dielectric layer 4010 is deposited over the caps 2018 at the topsof the relatively heavily-doped polysilicon islands 1716, resulting in astructure as shown in FIGS. 40A, 40B.

Thereafter, an array of pores are formed in the dielectric 4010, and thepores are filled with an electrode material to form bottom electrodes4032, with the result shown in FIGS. 41A, 41B, 41C.

Thereafter, a second array of openings 4220 can be formed through thedielectric 4110 of the structure shown in FIGS. 41A, 41B, 41C to exposeareas 4222 of the top surfaces of implanted relatively highly dopedcrystalline semiconductor material (1512, P+), resulting in thestructure illustrated in FIGS. 42A, 42B, 42C. The openings 4220 can beformed by etching, for example.

Thereafter, conductive plugs 4324 are formed by depositing conductivematerial in the openings 4220 in the structure illustrated in FIGS. 42A,42B, 42C, resulting in the structure shown in FIGS. 43A, 43B, 43C. Asmentioned above, the conductive plugs 4324 may be eliminated or usedonly occasionally is some embodiments.

Thereafter, strips 4430 of memory material overlain by strips 4434 oftop electrode material are formed over a construct as shown in FIGS.43A, 43B, 43C, resulting in a structure as shown in FIG. 44A, 44B, 44C.The strips are arranged so that they traverse the rows of bottomelectrodes 4032, and so that the memory material contacts the underlyingbottom electrodes. In the example shown, the top electrode 4434 alsoserves as a bit line.

FIGS. 45A-50B show stages in an embodiment of a process for formingmemory elements over access diodes having an island-type second regionthat is not self-aligned with the first diode region strip.

Beginning for example with a configuration as shown in FIGS. 25A, 25B,25C, a dielectric layer 4510 is deposited over the caps 2518 (labeled inFIG. 25A) at the tops of the relatively heavily-doped polysiliconislands 1316, resulting in a structure as shown in FIGS. 45A, 45B.

Thereafter, an array of pores are formed in the dielectric 4510, and thepores are filled with an electrode material to form bottom electrodes4532, with the result shown in FIGS. 46A, 46B, 46C. As mentioned above,the conductive plugs 4724 may be eliminated or used only occasionally issome embodiments.

Thereafter, a second array of openings 4720 can be formed through thedielectric 4610 of the structure shown in FIGS. 46A, 46B, 46C to exposeareas 4722 of the top surfaces of implanted relatively highly dopedcrystalline semiconductor material (1512, P+), resulting in thestructure illustrated in FIGS. 47A, 47B, 47C. The openings 4720 can beformed by etching, for example.

Thereafter, conductive plugs 4724 are formed by depositing conductivematerial in the openings 4720 in the structure illustrated in FIGS. 47A,47B, 47C, resulting in the structure shown in FIGS. 48A, 48B, 48C.

Thereafter, strips 4930 of memory material overlain by strips 4934 oftop electrode material are formed over a construct as shown in FIGS.48A, 48B, 48C, resulting in a structure as shown in FIG. 49A, 49B, 49C.The strips are arranged so that they traverse the rows of bottomelectrodes 4532, and so that the memory material contacts the underlyingbottom electrodes. In the example shown, the top electrode 4934 alsoserves as a bit line.

Thereafter, a dielectric fill 5010 is formed over the structure as shownin FIGS. 49A, 49B, 49C. An array of additional openings are formedthrough the dielectric fill 5010 to expose the surfaces of conductiveplugs 4724, and additional conductive plugs 5024 are formed bydepositing conductive material in the additional openings. Thereafter,word lines 5034 are formed over the dielectric fill 5010. The word linesare arranged so that they traverse the rows of additional conductiveplugs 5024, and so that the word line makes electrical contact with theunderlying conductive plugs. The resulting structure is shown in FIGS.50A, 50B. As mentioned above, the conductive plugs 5024 may beeliminated or used only occasionally is some embodiments.

FIGS. 51A-54C show stages in an embodiment of a process for formingmemory elements over access diodes having a second region in the form ofa pillar (as in FIGS. 27A, 27B). Beginning for example with aconfiguration as shown in FIGS. 39A, 39B, 39C, a dielectric 5110 isformed over the caps 3918 at the tops of the relatively heavily-dopedpolysilicon pillars 3916, and an array of tapered pores 5120 are formedthrough the dielectric 5110, exposing areas 5122 of the underlying caps3918. The resulting structure is shown in FIGS. 51A, 51B, 51C.

Thereafter, the pores are filled with a memory material 5232, with aresult as shown in FIGS. 52A, 52B, 52C.

Thereafter, strips 5330 of top electrode material are formed over therows of memory material elements 5232, and bit lines 120 b are formedover the top electrode strips. These strips may be formed by adeposition-mask-etch procedure, for example. The resulting structure isshown in FIGS. 53A, 53B, 53C.

Thereafter an additional dielectric fill 5410 is formed over thestructure of FIGS. 53A, 53B, 53C, and word lines 130 b are patternedover the structure, as shown in FIG. 54C, to which contacts are madeoccasionally as discussed above.

Other embodiments are within the following claims.

1. A memory cell, comprising an access device comprising a multilayerstack comprising: a first doped semiconductor region having a firstconductivity type in a single crystal substrate; an insulating layerover the substrate including a via; and a second doped semiconductorplug within the via and contacting the first doped semiconductor region,the second doped semiconductor region having a second conductivity typeopposite the first conductivity type, so that the first dopedsemiconductor region and the second doped semiconductor plug define a pnjunction; wherein the second doped semiconductor plug comprises apolycrystalline semiconductor, and the doping concentration of thesecond doped semiconductor region is more than 10 times higher than thedoping concentration of the first doped semiconductor region.
 2. Thememory cell of claim 1 further comprising a programmable resistancememory element over the insulating layer and coupled with the seconddoped semiconductor plug.
 3. The memory cell of claim 1 wherein thedoping concentration of the second doped semiconductor plug is in arange about 100 to about 1000 times higher than the doping concentrationof the first doped semiconductor region.
 4. The memory cell of claim 1wherein the first doped semiconductor region comprises a P-typesemiconductor and the second doped semiconductor plug comprises a N-typepolycrystalline semiconductor.
 5. The memory cell of claim 1 wherein thefirst doped semiconductor region comprises a N-type semiconductor andthe second doped semiconductor plug comprises a P-type polycrystallinesemiconductor.
 6. The memory cell of claim 1, further comprising anelectrically conductive cap on the second doped semiconductor plug. 7.The memory cell of claim 6 wherein the cap comprises a silicide.
 8. Thememory cell of claim 1, further comprising a barrier between the firstdoped semiconductor region and the second doped semiconductor plug. 9.The memory cell of claim 8 wherein the barrier comprises SiO₂.
 10. Thememory cell of claim 8 wherein the barrier comprises SiN_(x)O_(y). 11.The memory cell of claim 8 wherein the barrier has a thickness in arange about 5 to 25 Angstroms.
 12. A memory device comprising aplurality of doped semiconductor first access lines extending in a firstdirection, a plurality of second access lines extending in a seconddirection, a plurality of memory cells, each comprising an accessdevice, comprising a first doped semiconductor region having a firstconductivity type, wherein the first doped semiconductor region isformed in a single-crystalline semiconductor substrate and wherein thefirst doped semiconductor region is formed on a doped semiconductorfirst access line, an insulating layer over the substrate including avia; a second doped semiconductor plug having a second conductivity typeopposite the first conductivity type, the second doped semiconductorplug comprising a polycrystalline semiconductor, the first and seconddoped semiconductors defining a pn junction therebetween, and anelectrically conductive cap on the second doped semiconductor plug; anda memory material in electrical communication with the conductive cap,the memory material underlying and in electrical communication with acorresponding second access line.
 13. The memory device of claim 12wherein adjacent semiconductor first access lines are separated by atrench isolation.
 14. The memory device of claim 12 wherein the firstdoped semiconductor region comprises a lightly-doped P-typesemiconductor and the second doped semiconductor plug comprising a moreheavily-doped N-type polycrystalline semiconductor having a dopingconcentration higher than that of the lightly-doped P-typesemiconductor, and the cap comprises a silicide.
 15. The memory deviceof claim 14 wherein the first access line comprises a conductively-dopedP-type semiconductor.
 16. The memory device of claim 14 wherein thefirst access line comprises a conductively-doped P-type semiconductor,and including a conductive cap layer on surface portions of theconductively-doped P-type semiconductor.
 17. The memory device of claim12 wherein the first doped semiconductor region comprises alightly-doped N-type semiconductor and the second doped semiconductorplug comprises a more heavily-doped P-type polycrystalline semiconductorhaving a doping concentration higher that that of the lightly-dopedN-type semiconductor.
 18. The memory device of claim 17 wherein thefirst access line comprises a conductively-doped N-type semiconductor.19. The memory device of claim 17 wherein the first access linecomprises a conductively-doped N-type semiconductor, and including aconductive cap layer on surface portions of the conductively-dopedN-type semiconductor.
 20. The memory device of claim 16, furthercomprising a bottom electrode between the cap and the memory material.21. The memory device of claim 16, further comprising a barrier betweenthe first doped semiconductor region and the second doped semiconductorplug.
 22. The memory device of claim 21 wherein the barrier comprisesSiO₂.
 23. The memory device of claim 21 wherein the barrier comprisesSiN_(x)O_(y).
 24. The memory device of claim 21 wherein the barrier hasa thickness in a range about 5 to 25 Angstroms.
 25. A memory cell,comprising an access device comprising: a first doped semiconductorregion having a first conductivity type; and a second dopedsemiconductor plug having a second conductivity type opposite the firstconductivity type, the first and second doped semiconductors defining apn junction therebetween; wherein the first doped semiconductor regionis formed in a single-crystalline semiconductor, and the second dopedsemiconductor plug comprises a polycrystalline semiconductor; whereinthe second doped semiconductor plug has a doping concentration higherthan the doping concentration of the first doped semiconductor regionand the memory cell further comprises a programmable resistance memoryelement coupled with the second doped semiconductor plug.
 26. A memorycell comprising: a storage element, and a diode coupled as an accessdevice to the storage element, comprising a polysilicon node of a firstconductive type, a first single crystal silicon node of a secondconductive type, the polysilicon node comprising a polysilicon plugwithin a contact opening through a particular dielectric layer, andincluding a metal plug within another contact opening through the sameparticular dielectric layer.
 27. A method for making a memory celldriver, comprising providing a single-crystalline semiconductorsubstrate having a first conductivity type; forming a conductively dopedregion of a second conductivity type in an upper region of thesemiconductor substrate; forming a lightly-doped region of the secondconductivity type within and adjacent an upper surface of theconductively-doped region, the lightly-doped region having a dopingconcentration lower that that of the conductively doped region; formingisolation trenches defining a bottom driver region strip; depositing afirst dielectric material to fill the isolation trenches and cover thebottom driver region strip; planarizing to expose a surface of the firstdielectric material and the bottom driver region strip; depositing asecond dielectric material onto the surface of the first dielectricmaterial and the bottom driver region strip; forming a contact openingthrough the second dielectric material to expose an area of thelightly-doped region at the surface of the bottom driver region strip;forming a more heavily-doped polycrystalline material of the firstconductivity type in the contact opening, the more heavily-dopedpolycrystalline material establishing a pn-junction in the exposed areaof the lightly-doped region and having a doping concentration higherthat that of the lightly-doped region; and forming an electricallyconductive cap layer over a surface of the more heavily-dopedpolycrystalline material.
 28. The method of claim 27 wherein forming themore heavily-doped polycrystalline material of the first conductivitytype in the contact opening comprises a depositing the polycrystallinematerial by chemical vapor deposition to fill the contact opening,followed by planarizing by a chemical mechanical polish.
 29. The methodof claim 27 wherein forming the electrically conductive cap comprisesgrowing a silicide on the polycrystalline material in the contactopening.
 30. The method of claim 27 including forming a barrier layerbetween the more heavily-doped polycrystalline material and the exposedarea of the lightly-doped region.